The invention relates to a superconductor structure for conducting an electric current in a predetermined direction. The structure has at least the following parts: a support made from metallic material and a conductor track which is located on the support. The conductor track contains at least one interlayer which is deposited on the support and is made from electrically insulating material and at least one superconducting layer which is deposited on the interlayer and is made from a high-Tc superconductor material. The invention furthermore relates to a process for producing a corresponding superconductor structure. Such a structure and a corresponding production process is described in U.S. Pat. No. 4,921,833 (European published patent application EP 0 292 959. The invention furthermore relates to a current limiter device having such a superconductor structure.
Superconducting metal oxide compounds with high critical temperatures Tc of over 77 K are known and are therefore also referred to as high-Tc superconductor materials or HTS materials. The materials are particularly advantageous because the cooling in liquid nitrogen (LN2). Such metal oxide compounds include in particular cuprates based on special material systems, such as for example of the types Yxe2x80x94Baxe2x80x94Cuxe2x80x94O or Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O, in which the Bi component may be partially substituted by Pb. Within individual material systems there may be a plurality of superconducting high Tc phases which differ through the number of copper-oxygen lattice planes or layers within the crystalline unit cell and have different critical temperatures Tc.
Attempts are made to deposit these known HTS materials on different substrates for various applications. It is thereby a general requirement that the superconductor material with the maximum possible phase purity is sought. For example, in particular metal substrates are provided for conductor applications (see, example, the above-noted U.S. Pat. No. 4,921,833 and European publication EP 0 292 959).
In a corresponding superconductor structure for conductor applications, the HTS material is generally not deposited directly on a metal support strip serving as a substrate; rather, the support strip is first covered with a thin interlayer, which is also known as a buffer layer. The interlayer, with a thickness of the order of magnitude of about 1 xcexcm, is intended to prevent metal atoms from the substrate, which could impair the superconducting properties, from diffusing into the HTS material. At the same time, this interlayer can be used to smooth the surface and improve the adhesion of the HTS material. Appropriate interlayers generally comprise oxides of metals such as zirconium, cerium, yttrium, aluminum, strontium or magnesium and are therefore electrically insulating. In a discrete current-carrying conductor track, such as for example a strip conductor, this results in problems as soon as the superconductor changes to the normally conductive state at least in partial regions (known as quenching). This means that in sections the superconductor becomes resistive and thus adopts a resistance R, for example as a result of being heated beyond the critical temperature Tc (so-called hotspots). The current I which is being conducted in the superconductor then continues to flow through the superconductor material, the voltage drop U=Rxc2x7I forming only over the region which has become resistive. By contrast, in a metal substrate which is in strip form and supports the superconducting conductor track, the voltage U which is applied at the ends drops uniformly over the entire length of the conductor. Under certain circumstances, this may result in high voltage differences in the conductor track across the interlayer. Owing to the small thickness of the layer, this inevitably leads to electrical sparkovers and thus to the interlayer, and possibly the superconductor, being destroyed at certain points. A similar problem arises inter alia when superconductors of corresponding structure are used in magnet windings or in cables. This problem also arises in particular if resistive current limiter devices are produced using corresponding conductor strips. This is because in such a device the transition from the superconducting state to the normally conducting state is utilized to limit current in the event of a short circuit. In this case, it is impossible to provide the interlayer with a sufficient dielectric strength to withstand the operating voltages in the kV range which are customary for such devices.
In view of this problem, in a prior current limiter device (see, for example, commonly assigned German patent application DE 195 20 205 A), the HTS material is applied to electrically insulating, for example ceramic, substrate plates. It is also possible for an additional metal layer made from a material with a good electrical conductivity, such as Au or Ag, to be deposited directly on the HTS material as a shunt against burning through at the so-called hotspots. As a result, the HTS material is in electrically conductive surface-to-surface contact with the metal layer (see, for example, German patent DE 44 34 819 C). In that case, the above-mentioned voltage problem does not arise.
It is accordingly an object of the invention to provide a superconductor structure with high-Tc material, an associated production method and a current limiter device, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which has a reduced risk of undesirable voltage differences in the conductor track across the electrically insulating interlayer using a metal support.
With the foregoing and other objects in view there is provided, in accordance with the invention, an claim 1. superconductor structure for conducting an electric current in a predetermined direction, comprising:
a support of metallic material;
a conductor track on the support, the conductor track containing at least one interlayer of electrically insulating material deposited on the support and at least one superconducting layer of high-Tc superconductor material deposited on the interlayer;
at least one electrically conductive connecting part associated with the conductor track and extending in a current flow direction of the conductor track, between the superconducting layer of the conductor track and the support, the connecting part connecting the superconducting layer electrically in parallel with the support.
The advantages associated with this layout of the novel superconductor structure are that the metal support and the superconducting layer of the conductor track, as seen in the current-conducting direction, are brought into electrical contact with one another at least over a large part of the length of the structure, so that current can pass between them at least in that area. Consequently, sparkover through the interlayer is advantageously suppressed in the conductor track at the connecting regions.
In accordance with an added feature of the invention, the at least one connecting part is a strip-like metal layer extending laterally from the superconducting layer to an edge area of the support.
In accordance with an additional feature of the invention, the superconducting layer has an edge strip that laterally overlaps the interlayer. The edge strip forms the at least one connecting part and rests on a corresponding edge area of the support.
In accordance with another feature of the invention, the interlayer is formed with lateral recesses, where the superconducting layer rests on the support.
In accordance with a further feature of the invention, the support is made from Cu, Al, Ag, their alloys, or steel. In particularly, the support may be made from a NiMo alloy.
In accordance with again an added feature of the invention, the at least one interlayer is formed with a texture that is matched to the crystalline dimensions of the superconductor material.
In accordance with again an additional feature of the invention, the at least one interlayer is made from biaxially textured, yttrium-stabilized ZrO2 or CeO2.
In accordance with again another feature of the invention, the conductor track is a plurality of discrete conductor tracks and the support is a common support for the plurality of conductor tracks.
With the above and other objects in view there is provided, in accordance with the invention, a method for producing the above-outlined superconductor structure. In a specific feature, the at least one connecting part between the superconducting layer and the support is formed by avoiding a deposition of the interlayer in the at least one connecting region or removing the interlayer prior to the deposition of the superconducting layer.
In other words, the at least one electrically conductive connecting part between the superconducting layer and the support may be formed by avoiding, in the corresponding at least one connecting region, a deposition of the interlayer or the interlayer is removed prior to the deposition of the superconducting layer. Alternatively, it is also possible for at least one metal layer part which extends over the corresponding at least one connecting region to be deposited.
Moreover, the superconductor structure can also advantageously be produced by providing simultaneous thermal vapor deposition of the individual components of the superconductor material while oxygen is being supplied, or a laser ablation process, or a sputtering process, or a chemical vapor deposition process, in particular with organometal components of the superconductor material, or a screen-printing process as the deposition process for the superconductor material. The abovementioned deposition process can produce superconducting layers with a high critical current density on the interlayer.
Furthermore, it is advantageous if an ion beam assisted deposition (IBAD) process is provided for deposition of the interlayer material. This process can be used in particular to produce biaxially textured crystal structures of known interlayer materials. The superconductor material then advantageously also grows on in biaxially textured form, so that high critical current densities can be achieved by the absence of current-limiting grain boundaries.
Particularly advantageously, the superconductor structure according to the invention may be for the purpose of forming a current limiter device, because the use of a metal support for the superconductor material ensures rapid dissipation of heat to the cryogenic medium required and thus ensures a correspondingly rapid switching sequence.
In accordance with yet an added feature of the invention, a plurality of electrically interconnected modular current limiter elements are provided, where each has the above-outlined superconductor structure.
In accordance with yet an additional feature of the invention, each current limiter element has a winding comprising a bifilar-wound conductor strip having the superconductor structure.
In accordance with a concomitant feature of the invention, there are provided insulating strips between the strip conductor windings. The insulating strips are disposed to enable access for a coolant between adjacent conductor strip layers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a superconductor structure with high Tc superconductor material, process for producing the structure, and current limiter device having such a structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The structure and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.